In recent years, the use of cellular communication systems having mobile terminals which communicate with a hardwired network, such as a local area network (LAN) and a wide area network (WAN), has become widespread. Retail stores and warehouses, for example, may use cellular communications systems to track inventory and replenish stock. The transportation industry may use such systems at large outdoor storage facilities to keep an accurate account of incoming and outgoing shipments. In manufacturing facilities, such systems are useful for tracking parts, completed products and defects.
A typical cellular communication system includes a number of fixed base stations interconnected by a cable medium to form a hardwired network. The hardwired network is often referred to as a system backbone. Also included in many cellular communication systems are intermediate base stations which are not directly connected to the hardwired network. Intermediate base stations, often referred to as wireless base stations, increase the area within which base stations connected to the hardwired network can communicate with mobile terminals. Unless otherwise indicated, the term "base station" will hereinafter refer to both base stations hardwired to the network and wireless base stations.
Associated with each base station is a geographic cell. A cell is a geographic area in which a base station has sufficient signal strength to transmit data to and receive data from a mobile terminal with an acceptable error rate. The error rate for transmitted data is defined as the ratio of the number of transmitted data bits received in error to the total number of bits transmitted. It is economically inefficient to design a communications system with a "zero" error rate. Rather, depending on the requirements of users of the system, an acceptable error rate is determined. For example, an acceptable error rate may be set at a maximum error correcting rate capability of an error correcting code utilized by the system.
The shape of each cell is primarily determined by the type of antenna associated with a given base station. For instance, base stations which communicate with mobile terminals often have omnidirectional type antennas which provide for generally circular shaped cells and allow for a wide area of coverage. In many instances, however, the cell of a base station is not completely symmetrical because physical structures within the cell may partially block data signals emanating from the base station or create "dead spots" where no signals can pass. Further, the cell size may be decreased by machinery located in the vicinity of the base station which generates excessive noise levels that degrade a signal transmitted by the base station. Undesirable signals that interfere with the transmission and reception of a transmitted signal are collectively referred to as noise signals. A useful quantitative measure of relative noise in a communication system is the signal-to-noise ratio (SNR). The SNR is the ratio of the amplitude of a desired signal at any given time to the amplitude of noise signals at that same time.
Generally, when a mobile terminal is powered up, it "registers" with a base station through which the mobile terminal can maintain wireless communication with the network. In order to register, the mobile terminal must be within the cell range of the base station and the base station must likewise be situated within the effective cell range of the mobile terminal. It is generally not possible to have one base station service a large area by itself. This is due to transmission power restrictions governed by the FCC and the fact that the extra hardware needed to provide a mobile terminal with such a large cell range would add significantly to the size and weight of the mobile terminal thereby making it less desirable to use. Thus, cellular communication systems generally have several base stations spaced apart such that the collective cell area coverage of the base stations is sufficient to cover the entire area in which a mobile terminal may roam. As the location of the mobile terminal changes, the base station with which the mobile terminal was originally registered may fall outside of the geographic cell range of the mobile terminal. Therefore, the mobile terminal may "deregister" with the base station it was originally registered to and register with another base station which is within its communication range.
When designing a cellular communication system for a region, an appropriate number of base stations must be selected and their locations determined to assure cell coverage for the region. Each additional base station increases the cost of the communication system by the incremental cost of the base station itself and installation fees. When hardwiring a new base station to the network, both a data line and a power line must be provided. The data line allows the base station to transmit and receive information from the system backbone while the power line provides continual power to support the operations of the base station. Although wireless base stations do not require data lines since all data is communicated wirelessly, they do require power. However, providing power lines to wireless base stations can often be difficult. This is especially true in the common situation where a wireless base station is situated in a large outdoor storage facility having a concrete foundation, such as areas near a shipyard or loading dock. Typically, electrical outlets are not readily accessible in such areas and therefore power lines must be supplied to the wireless base station from the network or elsewhere. Power lines could be located on the surface of the concrete foundation, however, this provides an undesirable obstacle that must be avoided by heavy loading vehicles typically found operating at such facilities. Consequently, a trench is often created through the concrete in order to house the power lines. Unfortunately, providing such a trench adds a significant amount of extra time and cost to the installation process. Another method of supplying power to wireless base stations could involve suspending power lines from power poles. However, this method has been found implausible given the difficulty involved with erecting such power poles in the concrete foundation. As a result, there is a strong need in the art for a manner of supplying power to a wireless base station that is not unduly burdensome or costly.
Wireless communication systems such as those described above often involve spread spectrum (SS) technology. An SS communication system is one in which the transmitted frequency spectrum or bandwidth is much wider than absolutely necessary. Generally, SS technology is utilized for communications in the unlicensed bands provided by the FCC. These bands include the 902-928 MHZ and 2.4-2.48 GHz ranges in the U.S. The FCC requires that information transmitted in these bands be spread and coded in order to allow multiple user access to these bands at the same time.
One type of a digital SS communication system is known as a direct sequence spread spectrum (DSSS) system. The coding scheme of a SS digital communication system utilizes a pseudo-random binary sequence (PRBS). In a DSSS system, coding is achieved by converting each original data bit (zero or one) with a predetermined repetitive pseudo noise (PN) code. The PN code is used to convert effectively each data bit into a series of sub-bits often referred to as "chips". The rate of transmission of chips by a transmitter is defined as the "chipping rate". A type of PN code is illustrated in FIG. 1. For this example, the digital data signal 110 is made up of a binary "1" bit and a "0" bit. A PN code 120 representing the digital data signal 110 is comprised of a sequence of ten sub bits or chips, namely, "1", "0", "1", "1", "0", "1", "1", "1", "0", "1".
The digital data signal 110 is coded or spread by modulo 2 multiplying (e.g., via an "EXCLUSIVE NOR" (XNOR) function) of the digital data signal 110 with the PN code 120. If the data bit is a "1", then the resulting spread data signal (PN coded signal) in digital form corresponds to the PN code 120. However, if the data bit to be coded is a "0", then the spread data signal in digital form will correspond to a code 130. As can be seen, the code 130 is the inverse of PN code 120. That is, the PN code and its inverse are used to represent data bits "1" and "0" respectively.
A PN code length refers to a length of the coded sequence (the number of chips) for each original data bit. As noted above, the PN code length effects the processing gain. A longer PN code yields a higher processing gain which results in an increased communication range. The PN code chipping rate refers to the rate at which the chips are transmitted by a transmitter system. A receiver system must receive, demodulate and despread the PN coded chip sequence at the chipping rate utilized by the transmitter system. At a higher chipping rate, the receiver system is allotted a smaller amount of time to receive, demodulate and despread the chip sequence. As the chipping rate increases so to will the error rate. Thus, a higher chipping rate effectively reduces communication range. Conversely, decreasing the chipping rate increases communication range.
The spreading of a digital data signal by the PN code does not effect overall signal strength (or power) the data being transmitted or received. However, by spreading a signal, the amplitude at any one point typically will be less then the original (non-spread) signal.
It will be appreciated that increasing the PN code length or decreasing the chipping rate to achieve a longer communication range will result in a slower data transmission rate. Correspondingly, decreasing the PN code length or increasing the chipping rate will increase data transmission rate at a price of reducing communication range.
Additional methods of coding information in a SS system also exist. For example, in a frequency hopping (FH) system, data bits are spread and transmitted using a psuedo-random hop sequence. The hop sequence involves switching between different frequency channels in a given bandwidth. Only transmitters and receivers hopping on the same sequence are capable of communication with one another. Thus, multiple users can share the same bandwidths without significant interference by selecting different pseudo-random hop sequences with which to communicate. This is similar to DS systems where multiple users select different PN codes to avoid interference.
FIG. 1A schematically illustrates a transmitter system or assembly 100 of a DSSS system. Original data bits 101 are input to the transmitter system 100. The transmitter system includes a modulator 102, a spreading function 104 and a transmit filter 106. The modulator 102 modulates the data onto a carrier using, for example, a binary phase shift keying (BPSK) modulation technique. The BPSK modulation technique involves transmitting the carrier in-phase with the oscillations of an oscillator or 180 degrees out-of-phase with the oscillator depending on whether the transmitted bit is a "0" or a "1". The spreading function 104 converts the modulated original data bits 101 into a PN coded chip sequence, also referred to as spread data. The PN coded chip sequence is transmitted via an antenna so as to represent a transmitted PN coded sequence as shown at 108.
FIG. 1A also illustrates a receiver system or assembly, shown generally at 150. The receiver system 150 includes a receive filter 152, a despreading function 154, a bandpass filter 156 and a demodulator 158. The PN coded data 108 is received via an antenna and is filtered by the filter 152. Thereafter, the PN coded data is decoded by a PN code despreading function 154. The decoded data is then filtered and demodulated by the filter 156 and the demodulator 158 respectively to reconstitute the original data bits 101. To receive the transmitted spread data, the receiver system 150 must be tuned to the same predetermined carrier frequency and be set to demodulate a BPSK signal using the same predetermined PN code.
More specifically, to receive a SS transmission signal, the receiver system must be tuned to the same frequency as the transmitter assembly to receive the data. Furthermore, the receiver assembly must use a demodulation technique which corresponds to the particular modulation technique used by the transmitter assembly (ie. same PN code length, same chipping rate, BPSK). Because mobile terminals communicate with a common base station, each device in the cellular network must use the same carrier frequency and modulation technique.
An important aspect of any cellular communication system is the ability to optimize overall system performance. System performance may be optimized, for example, by communicating information at the fastest possible rates while minimizing packet errors, collisions and/or the need to re-transmit information among devices.
A drawback associated with current cellular communication systems is that PN code parameters such as PN code length and chipping rate must be selected to provide performance based on average communication range and average noise conditions. The data rate/range tradeoff leads to a cell size/throughput tradeoff in the communication system. The rate that each transmission occurs will limit the size of each cell. Thus, it would be desirable to have a cellular communication system wherein PN code parameter, modulation complexity and other transmitting and receiving parameters could be dynamically modified for each transmission based on distance between the transmitter and receiver and noise conditions such that an improved data transmission rate for that transmission could be achieved thereby enhancing system performance.
A further drawback associated with cellular communication systems is related to those systems utilizing wireless base stations. More particularly, the cell area of a wireless base station typically overlaps to a large extent the cell area associated with a base station hardwired to the network. Mobile terminals which roam into the overlapping areas transmit signals which are received or "heard" by both the base station and the wireless base station. This can occur despite the fact that the mobile terminal will typically be registered to only one of the devices, namely the mobile terminal is registered to communicate with either the base station or the wireless base station. Since many systems operate under a Collision Sense Multiple Access (CSMA) protocol, each base station (wireless and hardwired) and mobile terminal "listens" to the signal traffic in the air before transmitting information in order to avoid packet collisions. If the air is busy with signal traffic, the device performs a random back off in order to allow time for the air to clear. Unfortunately, in areas where there is overlapping cell coverage each transmission from a mobile terminal will cause the air signal traffic to be busy for both the wireless base station and the base station hardwired to the network. Thus, extra delays will occur due to the random backoff.
Similarly, the mobile terminals will also experience additional delays due to their internal random backoff protocol when attempting to transmit information in areas of contention. Further, even in systems not implementing CSMA or other similar protocol, there will be many times where two devices transmit information simultaneously. When this happens, both packets collide and each device must retransmit its packet, thereby also causing a loss in overall system performance.
Accordingly, it is also highly desirable to have a cellular communication system and method in which system performance can be optimized by eliminating unnecessary delays and packet collisions caused by overlapping cell areas of base stations and wireless base stations.